Following the 1957 election, he was appointed Postmaster General of Canada by John Diefenbaker. In that office he annoyed many of his colleagues by not permitting the post office to be used a reward for party supporters. He also drew ire by attempting to close many small and inefficient post offices.

He resigned as Postmaster General on July 12, 1962 after he was defeated in the 1962 election by Edmund Tobin Asselin. Hamilton could not speak French, and it was a more important issue in 1962 than it had been previously.

After his defeat Hamilton left Montreal for Vancouver where he ran a business.